The proposed research continues studies which were designed to elucidate the relationship between certain acute and chronic behavioral effects of d-lysergic acid diethylamide (LSD) and the activty of brain neurotransmitters such as serotonin (5-HT), dopamine (DA), and norepinephrine (NE). We will be concerned principally with a 2-lever choice situation in which the discriminative stimulus ("state-dependent") properties of LSD can be readily investigated although other behavioral paradigms will occasionally be used, e.g., bar-pressing on a single lever for water reinforcement delivered on a fixed-ratio schedule (FR 32), recording locomotor activity for 3-hr periods. In the 2-lever experiments, rats will be trained to detect the presence or absence of low (e.g., 10 micrograms/kg) medium (e.g., 80 micrograms/kg) or high (e.g., 240 micrograms/kg) doses of LSD (or of related compounds such as tryptophan, cyclazocine, quipazine, and phencyclidine) by responding for water reinforcement on either the left or right lever following drug and responding on the other (right or left) lever following saline. Ability to discriminate (detect the presence or absence of the drug state) will be tested twice weekly during 5-min extinction sessions (no water). These sessions will be preceeded by pretreatment with: 1) Drugs which either block or stimulate 5-HT receptors (inhibit raphe activity). 2) Drugs which act at DA receptors in the striatum and elsewhere. 3) Combinations of these and other agents. Hopefully, studies of this kind should elucidate further both the nature and the site of the LSD receptor(s).